1. Field of the Invention
This invention relates to the fabrication of semiconductor devices and, in particular, to the fabrication of III-V semiconductor devices.
2. Art Background
Many processes have been developed for the deposition of materials, e.g., semiconductor materials, on a substrate. One such process involves the use of a precursor gas, i.e., a gas that upon contact with the substrate undergoes a modification such as a chemical reaction to yield a deposited layer. The precursor gas is formed and flowed through a reactor tube to the deposition substrate. Although the precursor gas is generally formed by combining flows from multiple gas sources, typically there is only one combined precursor flow being generated during the deposition procedure.
The single precursor flow has the advantage of producing deposited layers having relatively uniform thickness on substrates of substantial size, i.e., having an area of 1 square inch or more. Generally, however, a semiconductor device such as a photodetector, e.g., a p-i-n photodetector, includes a plurality of sequentially deposited semiconductor layers of differing compositions. If a single precursor flow within a reactor is utilized, the composition of this flow must be changed for each subsequent layer deposition. Because gas flows cannot be immediately terminated or initiated, the region at the interface of sequentially deposited layers generally contains undesirable composition fluctuation between the two layer compositions. Undesirable fluctuation, in the context of this disclosure, is defined by comparing the peaks obtained by taking the X-ray diffraction rocking curve, as described by R. W. James in The Optical Principles of the Diffraction of X-Rays, Vol. II of The Crystalline State, Cornell University Press, Ithaca, N.Y., of the deposition structure, including the substrate and the desired epilayers. An epilayer deposition results in an undesirable composition fluctuation if the width of the X-ray rocking curve corresponding to this layer measured at 1/2 the peak height is greater than two times the width measured at 1/2 the peak height for the rocking curve corresponding to the substrate.
It is often desirable in semiconductor devices such as photodetectors to avoid undesirable interface composition fluctuations. Various techniques have been developed to accomplish this goal. Exemplary of these techniques is a process utilizing a dual precursor gas flow reactor such as shown in FIG. 1. Basically, the substrate, 20, is positioned at the orifice of a tube, 22, so that its major surface is perpendicular to the long axis of the tube. The first precursor gas flow, 25, is then directed along the tube, emerges from the tube, and contacts the substrate. If two such tubes are employed, then it is possible to establish a second precursor gas flow, 28, in the second tube before terminating or modifying the first flow. By a translational shift such as an eccentric rotation around an external shaft as shown at 26, the substrate is first subjected to one gas flow and then to the second at 27. In this manner, deposited layers having different compositions are sequentially formed on a substrate by a corresponding sequential exposure to the two flows.
Techniques utilizing such a plurality of precursor flows that simultaneously exist (at least during the period when the substrate is being transferred from one flow to another) advantageously produce transitional regions between layers with composition fluctuations that are less severe than that obtained by changing or modifying the precursor gas in the previously discussed single gas flow methods. However, as can be seen from FIG. 1, the geometry of the multiple precursor flow technique requires a relatively large reactor tube, 1, in comparison to the size of the substrate, 20. As a result, the quartz reactor tube is significantly harder to fabricate, much higher flow rates are required, and a substantially more sophisticated heating system is required to compensate for increased heat loss. Thus, for practical reasons, the use of a multiple precursor flow reactor has generally been limited to the deposition of epitaxial layers on substrates having an area substantially smaller than 1 square inch. Since it is now typically desired that production of devices such as p-i-n photodetectors be done by fabrication processes involving substrates having areas of 1 square inch and larger, reported multiple flow techniques have a significant limitation. Additionally, the thickness and composition uniformity of the deposited layer in a multiple precursor flow configuration is also generally significantly poorer than that achieved with single flow configurations. If an attempt is made to increase substrate area without increasing reactor size by depositing on a substrate whose major surface is positioned parallel to the longitudinal axis of the reactor tube, the already diminished uniformity is further decreased. Thus, for many applications, a multiple precursor flow reaactor is undesirable due to limitations on substrate diameter and deposited layer uniformity, while a single flow reactor generally leads to disadvantageous composition fluctuations between layers.
In addition to factors concerning thickness uniformity, substrate size, and composition fluctuations in the interface regions, it is desirable to control the purity of the deposited layer. (Purity is measured by the free carrier concentration of the layer in the absence of an intentionally introduced dopant.) The most commonly employed gas source employed in forming the precursor gas flow is produced by passing a gas over a liquid having a solid overlying crust. Use of such a two-phase body in producing the source flow generally yields excellent purity control but lacks excellent control over composition. (Control over composition is important in ternary and quaternary materials where a variety of stoichiometries between the constitutent elements exists but where, for a given application, only a narrow range of stoichiometries is acceptable.) The two-phase source generally involves a molten Group III element, e.g., gallium or indium, that has been saturated with a Group V material to produce a solid crust, e.g., a gallium arsenide or indium phosphide crust, over the molten liquid. For example, by flowing arsenic trichloride over molten gallium, a gallium arsenide crust is formed over the molten gallium, and species such as gallium chloride and As.sub.4 are formed by interaction of the two-phase source with the arsenic trichloride flow. These species are then typically combined with other species to produce the precursor gas flow. Although high purity deposited layers result, composition control depends on the maintenance throughout deposition of a crust with essentially constant dimensions relative to the underlying liquid. This maintenance is difficult to consistently achieve and leads to fluctuations in the composition of the precursor gas flow. (See Journal of Crystal Growth, 8, D. Shaw, page 117 (1971).) Solid source gas flows generated, for example, by passing arsenic trichloride over heated gallium arsenide or phosphorus trichloride over heated indium phosphide have also led to substantially reduced purity with no means of controlling the ratio of Group V to Group III elements in the final gas flow. (See Journal of Crystal Growth, 54, P. Vohl, pages 101-108 (1981).)
In contrast, a liquid source gas flow, e.g., a molten indium source subjected to a hydrogen chloride flow and combined with arsine and/or phosphine, has been utilized to avoid control problems. However, this approach generally leads to a relatively high level of impurity incorporation into the deposited layer, as noted by a relatively high free carrier concentration, i.e., greater than 10.sup.15 cm.sup.-3, in the absence of intentional doping. On the other hand, a liquid source gas flow, e.g., a molten indium/gallium alloy subjected to a hydrogen chloride flow that is combined with an arsenic trichloride flow, has been utilized to avoid impurity difficulties. By utilizing the liquid source gas flow, high purity for indium gallium arsenide has been reported. However, composition control is difficult since the ratio of gallium to indium must be maintained during each deposition and from one deposition to another by suitable replenishment of the consumed gallium. Additionally, the deposition rate for the resulting layer is quite low, e.g., less than 2 .mu.m/hour. (See Journal of Crystal Growth, 56, A. K. Chatterjee et al, page 591 (1982).) Thus, even if layer uniformity is achieved with limited composition fluctuations in interface regions, it is extremely difficult to obtain these attributes while additionally producing a high purity layer with controlled composition at an economic deposition rate.